The present invention relates to a press installation, such as that comprising a mechanical stamping, forming, shearing, punching or assembling press, wherein strip stock is feed off a coil through a straightener into the press, and more particularly relates to an apparatus for controlling the loop of strip stock between the straightener and the infeed side of the press.
In mechanical presses, the stock is often supplied in strip form and is fed into the press by means of an intermittently operating feed device such as a cam feed or the like. In order to permit substantially continuous movement of the stock from the reel, strip stock supply systems often maintain a supply loop in the stock ahead of the press feed so that the intermittent advancement of the stock is substantially isolated from the supply. Furthermore, straighteners are often provided between the coil of stock and the press feed in order to continuously feed the stock to the press feed and remove the set imparted to the stock because of its coiled condition. Since straighteners are generally continuous rather than intermittent in operation, the provision of a stock loop is necessary.
It is known to control the operation of the straightener so as to maintain the loop of stock between the straightener and the press at a substantially constant size, or within given upper and lower ranges of the loop size. One prior art technique for controlling the size of the stock loop is to provide a pair of limit switches, one located near the floor which initiates a stop feeding signal to the straightener when the loop becomes too large, and a second limit switch located above the loop, which causes the straightener to begin running when the loop gets too tight. Thus, the straightener is started and stopped as the loop becomes larger and smaller within the predetermined range established by the limit switches. Although this technique is the least expensive to implement, the stock is supplied in a jerky fashion which may result in a misfeed due to the stock being pushed or pulled at the input of the press feed. Additionally, it may result in premature wear of the straightener and the straightener control because the straightener is started and stopped intermittently, rather than being able to run continuously.
A second prior art technique for controlling the stock loop between the straightener and the press is a fairly complicated digital system which monitors the size of the loop and then feeds back signals to a digital processor that controls the motor drive for the straightener. Although this type of loop control can be very precise in maintaining the loop within a given size range, and will tolerate a fair amount of stock bounce, the system is quite expensive due to the sophistication of the circuitry. An advantage of a more complicated system of this type that is particularly adapted for high speed press operation as disclosed in U.S. Pat. No. 3,817,067 assigned to The Minster Machine Company.
The problem to be solved is the provision of a straightener control capable of maintaining the loop at a substantially constant size without the intermittent operation of the start/stop control discussed above yet without the higher cost of the more sophisticated digital control system. Additionally, it is desirable to cause the loop to increase to a maximum predetermined size greater than the normal `running` size when the loop press is stopped so that when the press is restarted, there will be sufficient material in the loop to enable the straightener to be started smoothly.